When using the Hummingbird plugins the menu on mobile is not showing. This is the answer I got from support from theme owner:
It looks like the caching/minifying plugin you’re using is included the parent style after the mobile style, which is incorrect.
The mobile style needs to be added after the parent style so that the CSS overrides the standard styling.
You might have to ask their support if you can specify the order in which CSS files are compressed into.